Sensor element for limiting current sensors for determination of the λ value of gas mixtures

ABSTRACT

A sensor element for limiting current sensors for determination of the λ-value of gas mixtures is proposed, which has at least one outer pumping electrode and one inner pumping electrode, on a solid electrolyte in platelet or foil form, conducting O 2-  ions, of which the inner pumping electrode is arranged in a diffusion channel for the measuring gas. It consists of a porous three-dimensional precious metal electrode having a supporting structure, the thickness of which electrode corresponds to the height of the diffusion channel. Characteristic of the inner pumping electrode is a greatly improved load-bearing capacity in comparison with electrodes of sheet-like design, whereby the service life of the sensor element is increased. A further advantage of the three-dimensional design of the inner pumping electrode is that it takes on a supporting function during lamination in the production of the sensor element by ceramic foil and screen printing techniques.

PRIOR ART

The present invention relates generally to limiting current sensors. Inthe case of such sensor elements, which operate on the diffusionlimiting current principle, the diffusion limiting current is measuredwith a constant voltage applied to the two electrodes of the sensorelement. In an exhaust gas produced in combustion processes, thiscurrent is dependant on the oxygen concentration as long as thediffusion of the gas to the pumping electrode determines the speed ofthe reaction taking place. It is known to construct such sensorsoperating on the polarographic measuring principle in such a way thatboth anode and cathode are exposed to the gas to be measured, thecathode having a diffusion barrier, in order to achieve an operation inthe diffusion limiting current range.

The known limiting current sensors serve as a rule for determination ofthe λ value of gas mixtures which designates the "total oxygen/oxygenrequired for complete combustion of the fuel" relationship of theair/fuel mixture burning in a cylinder, the sensors determining theoxygen content of the exhaust gas via an electrochemical pumping currentmeasurement.

On the basis of a simplified and inexpensive production method, inrecent years the production of sensor elements which can be produced byceramic foil and screen printing techniques has established itself inpractice.

Planar sensor elements can be produced in a simple and cost-effectiveway, starting with oxygen-conducting solid electrolytes in platelet orfoil form, for example from stabilized zirconium dioxide, which arecoated on both sides with an inner and an outer pumping electrode withassociated conductor tracks. The inner pumping electrode is in this caseadvantageously located in the edge region of a diffusion channel,through which the measuring gas is fed, and which serves as gasdiffusion resistance.

German Offenlegungsschrift 3,543,759, YAMADA, as well as EP-A 0,142,993,EP-A 0,188,900 and EP-A 0,194,082 also disclose sensor elements anddetectors, which have in common that they each have a pumping cell and asensor cell, which consist of oxygen-conducting solid electrolytes inplatelet or foil form and two electrodes arranged thereupon, and have acommon diffusion channel.

A disadvantage of sensor elements of the generic type of the main claimis that the front part of the inner pumping electrode, facing the fedmeasuring gas, is subjected to greater stress than the rear part of thepumping electrode, facing away from the fed measuring gas. This leads toa high electrode polarization, which requires a high pumping voltage.The latter in turn entails the risk of an electrolyte decomposition inthe region of the inner pumping electrode.

It is therefore proposed in German Offenlegungsschrift 3,728,618, in asensor element for limiting current sensors for determination of the λvalue of gas mixtures with outer and inner pumping electrodes arrangedon a solid electrolyte in platelet or foil form, conducting O²⁻ ions, ofwhich the inner pumping electrode is arranged on the solid electrolytein platelet or foil form in a diffusion channel for the measuring gas,as well as with conductor tracks for the pumping electrodes, to arrangein the diffusion channel on the side opposite the inner pumpingelectrode at least a second inner pumping electrode, which isshort-circuited with the first inner pumping electrode.

ADVANTAGES OF THE INVENTION

The sensor element according to the invention with the characterizingfeatures of the main claim has, in comparison, the advantage that, dueto the special design of the inner pumping electrode, its load-bearingcapacity is increased by virtue of its greater electrode area and theservice life of the sensor element is improved. A further advantagearises from the fact that the inner pumping electrode takes on asupporting function during lamination and compression in the case ofproduction of the sensor element by ceramic foil and screen printingtechniques.

The sensor element according to the invention can be used instead ofknown sensor elements of planar structure in limiting current sensors ofthe usual type. Coming into consideration in this case are broadbandsensors (λ 1) and lean sensors (λ>1). The sensor element according tothe invention can consequently be designed solely as a pumping cell, ifappropriate with a heating element, for example as a lean sensor fordiesel engines, and as such installed in a usual sensor housing, forexample of the type known from German Offenlegungsschriften 3,206,903and 3,537,051, and are used for measurement of the fuel/air ratio in alean exhaust gas. However, the sensor element according to the inventioncan also have, apart from the pumping cell, in addition a sensor cell(Nernst cell), which is provided with an additional air referencechannel and the one electrode of which is arranged in the region of thepumping electrode in the diffusion channel of the pumping cell and theother electrode of which is located in the air reference channel and isused for measurement of the fuel/air ratio in a lean or rich exhaustgas.

DRAWING

Advantageous embodiments of sensor elements according to the inventionare represented in the drawing.

FIG. 1 is a diagrammatic, greatly enlarged representation of a sectionthrough a sensor element according to the invention, which can beproduced by ceramic foil and screen printing techniques. It consistsessentially of the ceramic foils 1 and 2, onto which the innerthree-dimensional pumping electrode 3 and the outer pumping electrode 4have been printed, along with associated conductor tracks 5, 5', by thescreen printing method and which are laminated together, creating thediffusion channel 6 forming a tunnel, by means of a usual interlaminarbinder. In an advantageous way, the outer pumping electrode 4 and theassociated conductor track 5' are covered by a porous cover layer (notshown), for example of Mg-spinel.

The sensor element represented diagrammatically in FIG. 2 differs fromthe sensor element represented in FIG. 1 merely in that in the diffusionchannel 6 there is provided a porous filling 7, which serves asdiffusion barrier for the measuring gas.

FIG. 3 is a further diagrammatic, greatly enlarged representation of asection through another advantageous embodiment of a sensor elementaccording to the invention, which can be produced by ceramic foil andscreen printing techniques, in which the inner three-dimensional pumpingelectrode 3 and the outer pumping electrode 4 are arranged annularlyaround the measuring gas feed 8. It consists essentially of fourlaminated-together solid electrolyte foils 1, 2, 9 and 10 with thepunched-out measuring gas feed 8, the annular outer pumping electrode 4and the three-dimensional inner pumping electrode 3, arranged in thediffusion channel 6. In the case of the embodiment represented in FIG.3, the sensor element also has a heater 11. However, the foils 9 and 10with the heater are not absolutely necessary. The annular electrodes 3and 4 are connected to conductor tracks 5 and 5', which are insulatedwith respect to the solid electrolyte foils by means of insulatinglayers (not shown in the drawing), for example Al₂ O₃ layers. Theconductor tracks are connected to a voltage source (not shown), forexample a battery with a constant operating voltage in the range from0.5 to 1.0 volt. In an advantageous way, the outer pumping electrode 4and the associated conductor track 5' are in turn covered by a porouscover layer (not shown), for example of magnesium spinel.

Solid electrolytes which conduct oxygen ions and are suitable for theproduction of sensor elements according to the invention are, inparticular, those based on ZrO₂, HfO₂, CeO₂ and ThO₂. The use ofplatelets and foils of zirconium dioxide stabilized with yttrium (YSZ)has proved particularly advantageous.

In this case, the platelets and foils preferably have a thickness of0.25 to 0.3 mm.

The outer pumping electrode 4 preferably consists of a metal of theplatinum group, in particular platinum, or of alloys of metals of theplatinum group or alloys of metals of the platinum group with othermetals. If appropriate, the electrode may also contain a ceramicsupporting structure material, such as is also used for the productionof the inner pumping electrode. In contrast to the inner pumpingelectrode 3, designated here as "three-dimensional", the outer pumpingelectrode 4 however represents rather a more sheet-like two-dimensionalelectrode, i.e. an electrode of the usual type, which as a rule isthinner than the inner pumping electrode and preferably has a thicknessof 8 to 15 μm.

The inner three-dimensional pumping electrode 3 preferably consists of amixture of a metal of the platinum group, in particular platinum, or ofan alloy, as can also be used for producing the outer pumping electrode,and a supporting structure material, such as for example zirconiumdioxide stabilized with Y₂ O₃. If appropriate, the metal of the platinumgroup may be fully or partially substituted by an electron-conductivemetal or metal oxide, such as for example TiO₂ or perovskite, or by amixed-conductive (i.e. electrode (sic)--conductive and ion-conductive)metal oxide, such as for example CeO₂ or other oxides of rare earths aswell as mixed oxides such as for example uranium-scandium oxides. Theproportion by volume of supporting structure material is expedientlyaround 20 to 60%, preferably around 35 to 45%. It has also provedadvantageous if the degree of porosity of the inner pumping electrode isaround 10 to 40%. At least one part of the pores has in this casepreferably a pore diameter of greater than 1 μm. The average porediameter of the inner pumping electrode is in this case preferablyaround 2 to 10 μm. The porous supporting structure can in this case beproduced by the use of known pore-forming powders, which are added tothe mixture, for producing the electrode, of metal component andsupporting structure component as well as other usual additives, and areburned or vaporized during the production of the sensor element. Typicalpore formers which can be used with success are, for example,theobromine and carbon black as well as carbonates. The pore size of thesupporting structure can in this case be controlled by the particle sizeof the pore-forming powder used. However, the use of pore formers is notabsolutely necessary. Rather, it is also possible, for example, toproduce a porous supporting structure by use of a supporting structurematerial with comparatively low sintering activity. The compounds usedfor producing the electrodes can be prepared by usual known methods andapplied to, preferably printed onto, the solid electrolytes in plateletor foil form.

The inner three-dimensional pumping electrode 3 preferably lies directlyopposite the outer pumping electrode 4. It may cover the same area, asmaller area or greater area of the solid electrolyte than the outerpumping electrode 4. The inner pumping electrode 3 may accordingly onlyfill a comparatively small part of the diffusion channel 6 or acomparatively large part of the diffusion channel 6.

According to a particularly advantageous development of the invention,the inner pumping electrode 3 takes up only a part of the space of thediffusion channel 6 and a diffusion barrier 7 is arranged in thediffusion channel 6 ahead of the inner pumping electrode 3, asdiagrammatically represented by way of example in FIG. 2.

Such a diffusion barrier consists of a porous material, i.e. a materialwhich does not yet sinter compactly at the sintering temperature of thesubstrate, for example ZrO substrate, a material for example ofcoarse-grained ZrO₂, Mg-spinel or Al₂ O₃ with a grain size of, forexample, about 10 μm. To create an adequate porosity, if appropriatepore formers may be added, for example thermal carbon black powder,which bakes thoroughly in the sintering process, theobromine or ammoniumcarbonate. According to a particularly advantageous development of theinvention, a channel system for a mixed diffusion of Knudsen and gasphase diffusion, acting as a diffusion barrier for the measuring gas,may be arranged ahead of the inner pumping electrode 3. This channelsystem may, for example, consist of porously filled diffusion channelsfor a Knudsen diffusion and hollow channels for a gas phase diffusion.Such channel systems acting as diffusion barriers for the measuring gasare described in more detail in German Offenlegungsschrift 3,728,289.

For the purpose of improvement of the measuring accuracy, the diffusionbarrier may, in an advantageous way, contain platinum or a platinumalloy or another catalytically acting metal, in order to achieve anequilibrium adjustment of the gas going into the diffusion channel. Theproportion by volume of catalytically acting metal or metal alloy may bearound 10 to 90%. The diffusion barrier may take up the entire space ofthe diffusion channel left free by the inner pumping electrode 3 or onlya part of the same. Thus, for example, a free space may also remainbetween the inner pumping electrode 3 and the diffusion barrier 7. Likethe inner pumping electrode 3, the diffusion barrier 7 may be producedin an advantageous way by a thick-film technique.

The conductor tracks 5 and 5', belonging to the pumping electrodes 3 and4, preferably likewise consist of platinum or a platinum alloy. Pumpingelectrodes and conductor tracks may be applied to the solid electrolytebody by means of known methods, for example by screen printing. Betweenthe conductor track connecting the outer pumping electrode to a voltagesource (not shown in the drawing) and the solid electrolyte carrier, asa rule there is located an insulating layer, for example of Al₂ O₃. Itmay, for example, have a thickness of about 15 μm. The inner conductortrack is preferably insulated from the solid electrolyte carrier in asimilar way. The joining of the individual foils or platelets formingthe sensor element may take place by means of a method usual in ceramicfoil and screen printing techniques, in which the foils are broughttogether and heated to temperatures of about 100° C. In this case, thediffusion channel can be prepared at the same time. In an advantageousway, the latter is incorporated by a thick-film technique, for exampleby a theobromine screen printed layer, the theobromine being vaporizedduring the subsequent sintering process. Thermal carbon black powders,which bake thoroughly during the sintering process, or ammoniumcarbonate, which vaporizes, can likewise be used for example forproducing the diffusion channel.

If the diffusion channel is to have a porous diffusion barrier, insteadof a theobromine screen printed layer, for example a layer oftheobromine or of another vaporizable or combustible material and amaterial which does not yet sinter compactly at the applied sinteringtemperature of the solid electrolyte substrate, for examplecoarse-grained ZrO₂, magnesium spinel or Al₂ O₃ with a grain size of,for example, 10 μm, may be used.

EXAMPLE

For the production of a sensor element of the type representeddiagrammatically in FIG. 3, foils with a layer thickness of 0.3 mm ofzirconium dioxide stabilized with yttrium were used. The applying of theouter pumping electrode 4, consisting of platinum, and of the innerplatinum supporting structure electrode 3 to the carrier foils tookplace by the known screen printing technique, an about 20 μm thick, Al₂O₃ insulating layer having being applied in advance to the surface ofthe carrier foil 1 carrying the outer pumping electrode 4, in the regionof the conductor track 5' of the outer pumping electrode 4. Theconductor track 5 was also insulated with corresponding insulatinglayers. The annular pumping electrodes 3 and 4 had an outside diameterof 2.8 mm and an inside diameter of 1.4 mm with a thickness of the outerpumping electrode of 12 μm and of the inner pumping electrode of 40 μm.The inner pumping electrode was produced starting with a screen printingcompound which corresponded to the compound used for producing the outerpumping electrode, with the difference that it contained such a quantityof ZrO₂ stabilized with Y₂ O₃ that a degree of electrode porosity ofabout 30% was achieved. The conductor tracks were produced starting witha usual Pt cermet paste of 85 parts by weight of Pt powder and 15 partsby weight of YSZ powder. The diffusion channel 6 was incorporated by athick-film technique by a theobromine screen printed layer, thetheobromine being vaporized during the subsequent sintering process inthe temperature range of around 300° C., leaving behind an about 30 μmhigh and 1.3 mm deep annular gap. The central measuring gas feed orificehad a diameter of 0.25 mm. After the printing of the carrier foils, i.e.after applying the electrodes, conductor tracks, insulating layers and,if appropriate, a cover layer to the outer pumping electrode, the foils,once joined together, were subjected to a sintering process, in whichthey were heated for about 3 hours to a temperature in the range of1380° C.

For the production of a further sensor element with a heater, asrepresented diagrammatically in FIG. 3, before heating, further foilswere laminated to a printed-on heater.

The sensor elements produced were installed in the sensor housing of thetype known from German Offenlegungsschriften 3,206,903 and 3,537,051 andused for measurement of the fuel/air ratio in lean and rich exhaustgases.

We claim:
 1. Sensor element for limiting-current sensors fordetermination of the lambda value of exhaust gases of internalcombustion engines, which sensor element hasat least one outer (4)pumping electrode and at least one inner (3) pumping electrode on an O²-ion-conductive solid electrolyte, of which the inner pumping electrodeis arranged on the solid electrolyte in a diffusion channel (6) definedtherein for a measuring gas, said channel having a predetermined heightdefined perpendicular to a major planar surface of said solidelectrolyte, as well as conductor tracks for the pumping electrodes,wherein the inner pumping electrode (3) arranged in the diffusionchannel (6), consists essentially of a three-dimensional porouselectrode of a metal compound selected from the group consisting ofplatinum and a metal oxide compound, said inner electrode having asupporting structure and having a thickness sufficient to substantiallyfill the height of the diffusion channel (6).
 2. Sensor elementaccording to claim 1,wherein the height of the diffusion channel (6),and consequently the thickness of the inner pumping electrode (3), is ina range between about 30 and about 100 micrometers.
 3. Sensor elementaccording to claim 1,wherein the inner pumping electrode (3) comprises athree-dimensional platinum electrode having a supporting structure. 4.Sensor element according to claim 3,wherein the platinum in theelectrode is at least partially replaced by a metal compound which isconductive of at least electrons.
 5. Sensor element according to claim1,wherein the inner pumping electrode has a degree of porosity in arange between about 10% and about 40%.
 6. Sensor element according toclaim 1,wherein the average pore diameter of the inner pumping electrode(3) is in a range between about 2 and about 100 micrometers.
 7. Sensorelement according to claim 1,wherein a porous diffusion barrier (7) isarranged ahead, in the direction of diffusion of gas being measured, ofthe inner three dimensional pumping electrode (3) in the diffusionchannel (6).
 8. Sensor element according to claim 7,wherein the porousdiffusion barrier (7) consists essentially of platinum, for equilibriumadjustment of the gas mixture going into the diffusion channel (6). 9.Sensor element according to claim 1,wherein the sensor element isproduced by ceramic foil and screen printing techniques.
 10. Sensorelement according to claim 1,wherein an outer pumping electrode (4) isprovided and the outer pumping electrode and the inner pumping electrode(3) are arranged annularly around a measuring gas feed.
 11. Sensorelement according to claim 7,wherein the diffusion barrier (7) has achannel system for a mixed diffusion of Knudsen- andgas-phase-diffusion.